An emissive display device of a source current driving type displays an image using an organic diode, a quantum dot diode, an organic transistor or a quantum dot transistor and has advantages such as a light weight, a thin profile and a low power consumption.
However, the emissive display device of a source current driving type has disadvantages such as a deviation in a diode and a transistor caused by a fabrication process of a back plane, a distortion of white balance caused by a deviation in a material and a fabrication process of an emitting element and a crosstalk according to a voltage drop (IR drop) caused by a deviation in an operational load of a sub-pixel when driven by a source current.
To improve the disadvantages of the emissive display device of a source current driving type, several compensation methods have been performed during or after a fabrication process. All the conventional compensation methods use a lookup table according to experimental data under predetermined and limited conditions.
The conventional compensation methods use data of a lookup table which are a result of an experiment on a correlation between input and output. The results according to the conventional compensation methods may be changed by an error or a contribution of variables due to excessively complicate equations or excessively many related variables. In addition, the results according to the conventional compensation methods may be changed by an inconsistent fabrication process. As a result, the conventional compensation methods have been used within an allowable error range of the variables due to deviation in a fabrication process with a relatively low accuracy. (Japanese Publication No. 2005-295513, Korean Publication No. 10-2010-0118773)
Further, the conventional compensation methods using a lookup table may be used for a case where a parameter between input and output image signals is selected according to an amount of a current or a gray level voltage, or a case where a correlation between input and output images is deduced without using a parameter. (Japanese Publication No. 2008-129334)
Moreover, the conventional compensation methods using a lookup table may be used for a chromaticity calculation using a tri-stimulus value of an XYZ coordinate system of CIE (Commission International de l'eclairage) to obtain a target luminance, a target chromaticity and a target slope (gamma 2.2).
Although an amount of current consumption has a correlation (a proportional relation) to an amount of output luminance due to the nature of an emissive display device of a source current driving type, a difference in efficiency due to a deviation in process for each sample (display device) may not be a constant such that a proportional relation of a related current efficiency is fixed or may not be replaced by a simple function. As a result, a lookup table may be used within a smaller range of a specific gray level only for a specific sample having a current efficiency similar to that of a predetermined reference sample. (PCT International Publication No. WO 2009/028675, Korean Publication No. 10-2009-0093018, Korean Publication No. 10-2011-0023141)
In addition, when a chromaticity of a white is compensated using an XYZ coordinate system of a tri-stimulus value, a chromaticity calculation according to an exact red-green-and-blue (RGB) luminance may not be performed. As a result, a chromaticity calculation may be replaced by data of lookup table corresponding to state variation. (Korean Publication No. 10-2009-0109872, Korean Patent No. 10-1065406)
In the conventional compensation methods using a lookup table, a gray level voltage is not calculated through an exact functional equation. Instead, an approximate value of a relative variation with respect to a reference sample is calculated. As a result, a repetitive circulation compensation is required for convergence of the approximate value. Accordingly, a fabrication time increases and a productivity decreases due to a long time for compensation. (Korean Publication No. 10-2012-0108445, Korean Publication No. 10-2012-0028007, Korean Publication No. 10-2012-0108236)
In the conventional compensation methods using a lookup table, a structure of a ladder resistor including at least two columns have to be reversely calculated for finding out a gamma resistance value for a source current driving generating an input image signal. Theoretically, a calculation for infinite number of cases has to be performed to obtain output voltages of the first column from output voltages of the second column reversely. Further, a circulative compensation method such as measurement of luminance according to a determined structure of a ladder resistor and determination of a new structure of a ladder resistor has to be performed to make a luminance according to a final output voltage a target value. As a result, a long operation time through trial and error is required.
Accordingly, the conventional compensation methods using a lookup table has disadvantages in accuracy and operation time. Since a variable for each sample is not a constant proportional to a fabrication process but a relative approximate value with respect to a reference sample, accuracy decreases. In addition, since a repetitive process for convergence is required, a long time is taken for compensation. Further, since consistency of variation in efficiency is degraded due to a fabrication process for each sample, samples where a lookup table with respect to a reference sample is applied is limited. The above disadvantages function as a hindrance factor in improvement of yield of wider samples.